1. Field of the Invention
Exemplary embodiments of the present invention relate to light emitting devices having inorganic luminophores based on doped alkaline earth metal silicate compounds and quantum dot semiconductor compounds which are capable of converting high-energy primary radiation, i.e., for example, ultraviolet (UV) radiation or blue light, to a longer-wavelength secondary radiation within the visible spectral region, which can be used as radiation converters in light-emitting devices such as color or white-light emitting light emitting diodes (LEDs). Exemplary embodiments of the present invention also relate to light emitting devices having silicatic inorganic luminophores or quantum dot luminophores which may have improved stability to air humidity and other environmental factors, and increased operational lifetime.
2. Discussion of the Background
Luminophores may be used in light sources, such as LEDs that emit colored or white light. In an LED, luminophores, which may be used in combination with other luminophores, are used to convert ultraviolet or blue primary radiation emitted from an LED into longer wavelength secondary radiation, in particular, white light.
Alkaline earth metal silicate luminophores, which include the europium-doped alkaline earth metal orthosilicates, the corresponding oxyorthosilicates and the disilicates of the Ba(Sr)3MgSi2O8:Eu form, have been known for some time. An overview of the classification of the alkaline earth metal silicate compounds is documented by Hollemann-Wiberg, “Lehrbuch der Anorganischen Chemie” Inorganic Chemistry, 102 edition, (Walter de Gruyter & Co., Berlin, 2007). The preparation and the essential luminescence properties thereof have been described in detail in various patents and publications, for example: U.S. Pat. No. 6,489,716, issued to Tews, et al.; EP Appl. Pub. No. 0550937, applied for by Ouwerkerk, et al.; EP Appl. Pub. No. 0877070, applied for by Hase, et al.; and by W. M. Yen et al., “Phosphor Handbook”, 2nd Ed., CRC Press (2007). These publications indicate that such luminophores have high quantum and radiation yields for the conversion of high-energy radiation to visible light, and numerous representatives of this luminophore class, due to these properties, may be used in products for lighting, illumination, and display technology.
However, the luminophores based on the alkaline earth metal silicates also have various disadvantageous properties. Some of the disadvantages include a comparatively low radiation stability and high sensitivity of the luminophores to water, air humidity, and other environmental factors. The sensitivity depends on the particular composition of the luminophore, structural conditions, and the nature of activator ions of the luminophores. For some of the current applications of wavelength-conversion luminophores, these properties may be problematic. In view of the high lifetime demands, this may apply to LED applications. One known solution is to use suitable technologies and materials to generate (on the surface of pulverulent inorganic luminophores) barrier layers for reducing the influence of water vapour.
These processes may include encapsulation with organic polymers, coating with nanoscale oxides such as SiO2 or Al2O3, or chemical vapour deposition (CVD) of such oxides. However, in relation to silicatic luminophores, the protection achievable may be insufficient to improve the lifetime of corresponding LED lamps to the desired degree. Furthermore, in the case of coated luminophores, it may be necessary to accept losses in brightness, shifts in the color location, and other quality losses. Processes for microencapsulation of the luminophore particles by means of gas phase processes may be inconvenient and costly.
Quantum dots also can be used as luminophores and their wavelengths become shorter as their sizes become smaller. Quantum dots are very reactive due to their small size, and thus, quantum dots have also a comparatively low radiation stability and high sensitivity to water, air humidity, and other environmental factors.